The present invention relates generally to a gas turbine engine stator case and more particularly, to a stator case having transient deflections matched to a rotor of the engine.
Gas turbine engines have a stator and one or more rotors rotatably mounted on the stator. The rotors have blades arranged in circumferential rows. Each of the blades extends outward from a root to a tip. The stator is formed from one or more tubular cases which house the rotor such that the rotor blades rotate within the cases. In the compressor section of gas turbine engines, it is desirable to minimize clearances between the blade tips and interior surfaces of the cases to improve engine stall margins and efficiencies. In the turbine section of gas turbine engines, it is desirable to minimize clearances between the blade tips and interior surfaces of the cases to improve engine efficiency.
The clearances between the rotor blade tips and the interior surfaces are determined by the deflections of the blade tips and the deflections of the interior surfaces of the stator cases. The deflections of the blade tips are caused by mechanical strain due to centrifugal forces on the spinning rotor and thermal growth due to elevated flowpath gas temperatures. Likewise, the deflections of the interior surfaces of the cases are a function of mechanical strain and thermal growth. These deflections may be adjusted by controlling mechanical strain and thermal growth of the rotors and stator cases. In general, it is desirable to adjust the deflections so the clearances between the rotor blade tips and the interior surfaces of the stator cases are minimized, particularly during steady state engine operation.
In the past, the stator case deflection has been primarily controlled by directing cooling air to portions of the case to reduce deflections thereby reducing clearances between the blade tips and the interior surfaces of the cases. Alternatively, circumferential ribs were formed in the case directly above the blade tips to reduce stator deflections. However, there is a need to reduce clearances further to improve stall margins and efficiencies of gas turbine engines.
Among the several features of the present invention may be noted the provision of a stator case for a gas turbine engine having a stator and a rotor. The rotor has a plurality of circumferential rows of blades. Each blade extends radially outward from a root to a tip. The case includes a tubular shell extending axially between a forward end and an aft end. The shell has an interior surface defining a hollow interior sized and shaped for receiving at least a portion of the rotor of the gas turbine engine. The case also includes a circular forward flange extending radially outward from the forward end of the shell and a circular aft flange extending radially outward from the aft end of the shell. In addition, the case includes a circular rib extending radially outward from the shell between adjacent rows of blades. The rib is sized and shaped for adjusting transient deflections of the shell to generally match transient deflections of the tips of the plurality of rotor blades to reduce a transient clearance between the interior surface of the tubular shell and the tips of the rotor blades.
In another aspect, the present invention includes a gas turbine engine comprising a stator and a rotor rotatably mounted on the stator. The stator includes a case as described above.
Other features of the present invention will be in part apparent and in part pointed out hereinafter.